CHAPITRE 6 CONCLUSIONS
6.2 Recommandations pour travaux futurs
• L’algorithme d’identification et évaluation des ponts peut être inclus dans un logiciel. L’approche d’optimisation numérique peut être utilisée pour identifier la topologie du réseau d’échangeurs de chaleur résultant de modifications pontales.
• La méthode pontale peut être appliquée et testée pour l’intégration de systèmes traversés par plusieurs formes d’énergie. Le rapport exergie/énergie peut remplacer la température afin de représenter la dégradation en général. Ceci permettrait par exemple d’analyser la production d’électricité dans une turbine et son intégration dans un site industriel.
• Le principe pontal peut être utilisé pour améliorer d’autres approches d’intégration énergétique basées sur l’analyse de procédé ou sur l’optimisation. Les principes de la méthode pontale peuvent être adaptés pour l’intégration massique. Ils pourraient être étendus afin d’analyser d’autres systèmes traversés par des flux respectant les principes de conservation et dégradation.
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ANNEXES
Annexe 1: Article - Bridge Analysis to Reduce the Industrial Energy Requirements by Heat Exchanger Network Retrofit: Part 1 - Concepts
Annexe 2: Article - Bridge Analysis to Reduce the Industrial Energy Requirements by Heat Exchanger Network Retrofit: Part 2 - Applications
Annexe 3: Article - Application of Bridge Analysis to Retrofitting Direct- and Indirect- Contact Exchange networks
Annexe 4: Article - Energy Transfer Diagram for improving integration of industrial systems Annexe 5: Rapport - Formulation pour identifier la topologie du réseau résultant de
ANNEXE 1
Article
Titre: Bridge Analysis to Reduce the Industrial Energy Requirements by Heat Exchanger Network Retrofit: Part 1 – Concepts
Auteurs : J.-C. Bonhivers, S. Bala et P. R. Stuart Journal : soumis à Energy.
Bridge Analysis to Reduce the Industrial Energy Requirements by Heat-
Exchanger Network Retrofit: Part 1 - Concepts
Jean-Christophe Bonhivers, Balasubrahmanyan Srinivasan, Paul R. Stuarta
a
NSERC Chair in Design Engineering, Department of Chemical Engineering, Ecole Polytechnique de Montreal, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec H3C 3A7, CANADA
E-mail: paul.stuart@polymtl.ca
Abstract
Energy savings in existing plants usually have positive economic and environmental impacts. Mathematical approaches to heat-exchanger network (HEN) retrofit are usually complex and do not guarantee identification of the global optimum. Thanks to its simplicity the pinch-based approach is widely used, even though difficulties for its adaption to HEN retrofit are encountered. The energy supplied is degraded throughout the industrial process. Reducing heat consumption implies a reduction in the flow rate of heat transferred from the heating utility through the existing heat exchanger network until rejected to the environment. This progressive transfer of heat in the existing heat exchanges from the heating utility to the environment is not explicitly analyzed in the present approaches for HEN retrofit. This paper presents the concepts supporting a method for HEN retrofit. The fundamental set of modifications necessary to reduce the heat consumption is made explicit and is represented by a “bridge”. A method to enumerate the bridges is described. A heat transfer diagram to identify bridges is also presented. Finally a network table to easily identify and evaluate bridges is proposed. A global procedure for HEN retrofit and case studies are presented in a second paper.
1. Introduction 1.1. Problem context
The manufacturing process industries are large energy users. Energy analysis methods for new and existing plants have been developed in last decades to increase their profitability and reduce the environmental impacts of their activities. Energy analysis can involve the following areas in a plant: (1) the utilities which include the production of heat and electricity, and the cooling, (2) the heat exchanger network (HEN), and (3) the process operations. This paper presents concepts to reduce the heat consumption by HEN retrofit. However these concepts can also be applied for energy analysis of utilities and process operations in new or existing plants.
1.2. Literature review about HEN retrofit methods
Heat integration, since its development in the late 1970s, has demonstrated large energy reductions across numerous industry sectors. The different approaches used in heat integration can be broadly classified as either (i) insight-based methods or (ii) optimization-based methods. Insight-based methods use graphical tools such as composite curves to calculate energy targets and heuristics for network design to achieve these targets [1-5]. The main advantages of these methods are their simplicity, their graphical representation of the problem, and the involvement of the designer throughout the steps. Optimization-based methods use mathematical programming to minimize or maximize an objective function such as cost or profit with respect to constraints such as energy balances and heat-exchange models. However, optimization approaches for retrofitting heat- exchanger networks are complex and do not guarantee identification of the global optimum. Pinch- based approaches, which do not guarantee to find the global optimum either, are widely used for heat integration due to the simplicity of the concepts.
Pinch analysis for thermal energy reduction has been widely used throughout the refining and petrochemical industries for several decades. Other industries such as chemicals, pulp and paper, and the food and beverage industries have also benefitted from pinch analysis. However, energy savings in such industries often require consideration of the interactions between mass and heat networks. Therefore, much recent progress in process integration has been focused on developing methodologies to combine heat integration and mass/water integration. The classical thermal pinch analysis technique was initially developed to analyze indirect-contact heat exchange in networks for energy use reduction and has evolved over recent decades to address key industry energy analysis and design needs such as (a) network retrofit, (b) stream mixing, and (c) site-wide energy analysis. Thermal pinch analysis has served these industry sectors well, even though many believe that thermal pinch analysis requires considerable experience, especially with regard to data extraction.
Although pinch analysis has been successfully used in retrofit situations, difficulties are encountered when considering constraints specific to each connection, especially if the network involves both direct- and indirect-contact heat exchanges. Pinch analysis requires summing of the heat loads in each temperature interval to obtain the composite curves and then setting a minimum temperature difference between these curves to identify the pinch temperatures and the minimum energy requirement. Consequently, the minimum energy requirement and the pinch temperatures, which
divide the network into two parts, are evaluated without considering constraints specific to each connection. Even in the ideal case for the application of pinch analysis, i.e., a network where all the connections are practically feasible and involve only indirect-contact heat exchanges, dividing the network into portions above and below the pinch by setting a common minimum temperature difference for streams with different heat-transfer coefficient values is a significant assumption. Pinch analysts sometimes include in the definition of each source and sink a value “DTmin/2”, which represents the stream’s contribution to the minimum temperature driving force. The composite curves of the process are computed using the value of the DTmin/2 contribution from each stream. This makes it possible to account for the different heat-transfer coefficient values. This approach is an improvement over using a global temperature difference; however it still does not allow temperature constraints to be set in accordance with the type of heat exchange taking place. Most industrial processes include both indirect-contact heat exchanges, where a minimum difference in stream temperature expresses the trade-off between energy savings and capital spending for